This invention relates to the automatic identification of audio signals, particularly broadcast audio signals.
It is often desirable to be able to produce a log of what audio signals are broadcast and when they are broadcast. This information is particularly useful to companies who pay for commercials advertising their goods or services. Using this information, a company is able to monitor how often and at what time their commercials are broadcast within a given period of time. They can thus monitor the broadcasts to ensure that they are getting what they pay for.
It will be appreciated that the term xe2x80x9caudio signalxe2x80x9d encompasses both analog and digital signals.
It is also useful to have a record of the times particular audio cuts were broadcast for legal purposes. For example, if a particular audio cut is being used as evidence in a court, an accurate time of broadcast may be obtained.
Owners of copyright in audio cuts would also be keen to have a record of when and how often their song, for example, is broadcast, for the purposes of collecting royalties.
Methods already exist to keep logs of broadcast patterns. One such method is a purely manual one in which one or several human operators physically monitor all broadcasts by watching a television set or listening to a radio. One television set and one radio must be monitored for each broadcast frequency. This is a labour-intensive and often inaccurate method of logging broadcasts.
Automatic methods do exist, however, these have their own disadvantages. Some of these methods tag a piece of audio in some way with identifying data, however, this data sometimes interferes with the audio signal, or is detectable as an audible signal over the top of the original audio signal. For many broadcast situations, this is an unsatisfactory outcome. Furthermore, audio signals often undergo heavy audio processing during the journey from transmitter to receiver. Often the signal is passed through a sub-band coded link (e.g. MPEG satellite ), and/or multi-band limiting. In many cases, the identification data signal imposed on the audio signal is unable to survive this processing and cannot be effectively detected and/or retrieved upon reception.
It is therefore an object of the invention to provide an improved means and method of automatically identifying an audio signal, in which the identifier is more reliable and robust than prior methods, but which does not substantially interfere with perceived audio quality.
In a broad form of the present invention, there is provided a method which includes:
A. removing a band of frequencies centred at a predetermined notch frequency from said audio signal;
B. spectrally shaping said data signal such that it takes on the precise shape and magnitude of the envelope of the audio signal at said removed band of frequencies centred at said notch frequency; and
C. inserting said shaped data signal into said audio signal within the removed band centred at said notch frequency.
The data signal will preferably include a carrier signal modulated to enclose data using minimum shift frequency shift keying (MSK). Preferably, the notch frequency will be at approximately 3 kHz. The data signal will, in a preferred embodiment, be present over substantially the entire timespan of the audio segment comprising the audio signal. The data may include two six-digit numbers presented in binary form as a 40-bit field and will preferably represent an identification tag.
According to a second aspect of the invention, there is provided a method of detecting a data signal inserted into an audio signal according to the first aspect, the method including:
A. receiving said tagged signal at a receiving station;
B. band pass filtering said received signal to extract said inserted modulated data signal; and
C. removing the amplitude modulation resulting from the spectral shaping from said modulated data signal.
According to a third aspect of the present invention, there is provided a method of identifying a transmitted audio signal, the method including the steps of:
A. removing a band of frequencies centred at a predetermined notch frequency from said audio signal;
B. spectrally shaping an identification signal identifying a particular audio segment such that it takes on the precise shape and magnitude of the envelope of the audio signal at said removed band of frequencies centred at said notch frequency;
C. inserting said identification signal into said audio signal to produce a tagged signal;
D. transmitting said tagged signal;
E. receiving said transmitted tagged signal;
F. bandpass filtering said received tagged signal to extract said identification signal;
G. removing the amplitude modulation resulting from the spectral shaping from said extracted identification signal; and
H. reading and/or recording said identification signal to identify said tagged signal.
According to a fourth aspect of the present invention, there is provided an encoder for encoding a data signal onto an audio signal, the encoder including:
filter means for removing a band of frequencies centred at a predetermined notch frequency from said audio signal;
shaping means for spectrally shaping said data signal such that it takes on the precise shape and magnitude of the envelope of the audio signal at said removed band of frequencies;
inserting means for inserting said shaped data signal into said audio signal within the removed frequency band centred at said notch frequency; and
data input means for receiving data to be encoded into said audio signal.
According to a fifth aspect of the invention, there is provided a decoder for decoding an encoded audio signal encoded by the encoder of the invention, the decoder including:
a receiver input for receiving said encoded audio signal;
receiver filter means for extracting a band of frequencies containing said code from said encoded audio signal;
means for removing the envelope modulation applied to said data signal; and
receiver demodulation means for demodulating said data signal.
The present invention thereby provides a method and apparatus for inserting and detecting a data signal into an audio signal such that the data signal is virtually inaudible by a listener of the audio signal, yet is robust enough to survive severe audio processing.
This is accomplished by inserting the data signal into a notch created in the audio signal, and spectrally shaping the inserted data signal to conform precisely to the envelope of the audio signal at the frequency band at which the data signal is inserted.